The immediate specific aim of this study is to determine the diagnostic accuracy of interpretation of subtraction images which is obtainable by use of state-of-the-art image analysis systems. It is proposed to achieve the stated specific aim by comparing the observer performance for the interpretation of subtraction images using the different image analysis systems. Focus will be directed to three areas of study: 1. technics for standardizing x-ray beam angulation 2. comparison of electronic with film-based methods for image acquisition. 3. after the fact minimization of projection angle discrepancies of subtraction images generated from radiographs differing in projection geometry. 1.Technics for standardizing x-ray beam angulation: Three methods for obtaining radiographs believed to be suitable for subtraction radiography will be investigated for their contribution of diagnostic accuracy: a. a rigidly fixed stent based system, b. an uncoupled stent based system, c. a non-stent based system using extra-oral fixation. Angle discrepancy of the x-ray tube and subject at a 6 months time interval will be determined by deflection of a light beam from a mirror reproducibly positioned on the patient. The mirror will be so positioned by means of a device fitting on to 2 adjacent bracketed teeth. 2. Comparison of electronic with film-based methods for image acquisition: An ancillary, but nevertheless very important component of this study is to compare the diagnostic accuracy for a film-based system using rectangular collimation to an electronic sensor (Trophy) system. An overriding goal is to obtain images with minimum radiation exposure to the patient. 3.Improvement of diagnostic accuracy due to misangulation error: We will attempt to test these three types of systems at their optimal potential. However, since no system is anticipated to function perfectly we propose to develop algorithmic methodologies to ameliorate slight misangulation errors and test whether these will enhance accuracy of interpretation of subtraction images.